An optical information reproduction device is a device that reproduces information that has been recorded on an information recording medium such as a CD (compact disk), DVD (digital versatile disk), or other such optical disk, an optical card memory, or the like. There are also devices that further comprise a function of recording information onto such information recording media (hereinafter optical information reproduction devices that also comprise a recording function will sometimes be referred to as optical information recording and reproduction devices).
Information recording media with a multilayer structure, in which a plurality of recording layers are stacked over one another, have been provided as information recording media capable of recording information three-dimensionally in order to further increase capacity. FIG. 7 illustrates an example of a conventional optical information recording and reproduction device that includes an information recording medium with such a multilayer structure (see, for example, Yoshimasa Kawata, “Three-dimensional Optical Memory Using a Femtosecond Laser,” Optronics, pp. 138-142 (2001)).
With the information recording medium 121 shown in FIG. 7, a recording unit 103 is provided on a glass substrate 104, and the recording unit 103 is formed by alternating recording layers 101a to 101d with intermediate layers 102a to 102c. Specifically, the information recording medium 121 is capable of recording information three-dimensionally because a plurality of recording layers are stacked in the optical axis direction (the Z axis direction in the drawing) of an objective lens (the objective lens 106 for focusing recording light or reproduction light on the recording layers).
A recording light source 120a and a reproduction light source 120b are provided in the conventional optical information recording and reproduction device shown in FIG. 7. The recording light source 120a is a titanium sapphire laser. The recording light 122a emitted from this light source 120a, which has a wavelength of 790 nm and a high peak power, goes through a beam splitter 118a, the beam diameter is expanded by a beam expander 123, and the beam then goes through a beam splitter 118b and is focused (as converged light 107) by an objective lens 106 on the target recording layer (the recording layer 101c in the drawing) of a multilayer information recording medium 121 capable of three-dimensional recording. Information is recorded by forming recording pits 105 as recording marks in the recording layer 101c by utilizing the nonlinear absorption (such as two-photon absorption) that occurs in the focusing unit.
The reproduction light source 120b is a helium-neon laser. The reproduction light emitted from this light source 120b, which has a wavelength of 0.6328 μm and a low peak power, is bent in the −Z axial direction by the beam splitter 118a, after which it goes through the same optical path as the recording light 122a and is focused on the target recording layer (the recording layer 101c in the drawing). The light reflected by the recording pits 105 is bent in the Y axial direction by the beam splitter 118b and focused on a detection lens 111. A pinhole plate 114 having a pinhole is disposed at the focal point of the detection lens 111, and a signal can be reproduced by detecting light that has passed through the pinhole with a photodetector 119.
FIG. 8 shows the XY plane perpendicular to the Z axis, and schematically illustrates the recording pits 105 formed in one recording layer. The recording pits 105 are hatched in this drawing. The squares shown by broken lines are non-recording pits. The recording pits 105 are square, the mark length, which is the pit size in the Y axial direction (labeled ML in the drawing) is 0.5 μm, and the track pitch (labeled TP in the drawing) is 1 μm. In the drawing, the Y axial direction is the track direction, and the X axial direction is perpendicular to the track direction.
As shown in FIG. 8, the recording of information with the above-mentioned conventional optical information recording and reproduction device is accomplished by a mark position recording method, in which the size of the recording pits is consistent and information is recorded according to whether or not a pit is formed. Mark length recording, meanwhile, is a recording method that affords a greater recording capacity than this mark position recording. Mark length recording is a method in which recording is performed by varying the length of the recording marks, and affords a recording capacity of about two times or more as compared to mark position recording.
However, with the above-mentioned conventional device, in the reproduction of information that has been recorded to a target recording layer (such as the recording layer 101c shown in FIG. 7), light is lost through diffraction loss of the converged light 107 due to the recording pits formed in the recording layers 101a and 101b located in front of the target layer (on the incident light side, and the objective lens 106 side), and this adversely affects the reproduction of information. Specifically, when there are many recording layers in front of the target layer, the light is attenuated by the time it reaches the target recording layer, making reproduction impossible, so it has been difficult to reproduce information from large-capacity information recording media having many layers.